Some apologists apparently believe that everyone really believes in God, and anyone who claims not to believe is simply lying, or angry at God. They may claim that atheism is a form of rebellion, or that God is so indelibly written on everyone's heart that no one can deny belief. The argument is frequently supported by the claim that it is impossible to know definitively that there is no God, therefore an atheist must be an agnostic. This argument is fallacious as it incorrectly assumes that atheism and agnosticism are mutually exclusive.

A Lack of Evidence

This claim has a lack of evidence, and seems to be based on pure speculative assumption.
How do we know that theists and religious people aren't really just atheistic nonreligious people who are just lying and merely angry at reality?
The the reason it is known they really do believe this stuff is because of how far they are willing to go to defend it. From Muslims who blow themselves up, to Christians, Hindus, and other such people who donate their life savings to clergy in the hopes of reaching eternal life, Nirvana, etc.
By contrast, most atheists don't even care to even call themselves atheists.
According to the Pew forum Survey, even though about 1.6% US Americans self identify as Atheists (http://religions.pewforum.org/affiliations). However, 5% of the USA population in 2007 didn't believe there was a god, while 3%, weren't sure (http://religions.pewforum.org/comparisons#)
Both are qualifications for being atheist--lack of a belief in a deity, or deities. This implies that as many as 8% of US Americans are technically atheists, yet don't even know, or care enough to call themselves atheists.
They could also be afraid of persecution from the Christian Majority, However.